The present invention relates to a data recording and playback apparatus having a record/playback head which writes various information into and reads them out of tracks formed in an optical disc, magnetic disc or like rotary disc. More particularly, the present invention relates to a positioning device capable of moving the record/playback head along the surface of a disc to accurately position it relative to a desired track on the disc for the purpose of recording or playing back data.
A major achievement in the realm of recording and playback technology today may be optical discs which were evolved from video discs. In fact, there have been developed optical disc apparatuses which use various kinds of optical discs such as audio discs, mass image files and disc memories for large capacity computers, in addition to video discs.
Usually, an optical disc for use with an optical disc apparatus is formed on its surface with a number of concentric information tracks in order to record and playback various information, To write or read information into or out of such an information track, a disc is rotated and a record/playback head is moved radially of the disc for a tracking control. For the tracking control, the record/playback head has to move with an accuracy which is at least on the order of submicrons. Such an accuracy of head movement is indispensable due to the fact that pits in adjacent tracks on the disc surface are spaced by microns, that is, an accuracy higher than this is needed for precise tracking. Meanwhile, it is necessary to move the record/playback head in the radial direction of the disc in the course of the tracking control and, therefore, to reduce the dimensions of the whole head.
A prior art record/playback head carries therewith optics which usually includes an object lens. During the tracking control, the object lens is displaced not only in the focal direction but also in the radial direction. The radial movement of the object lens, however, tends to effect the optical axis of the optics and thereby increase the error while data is written into or read out of the disc. This limits the allowable displacement of the object lens in the radial direction and thereby requires the disc itself to be produced with a significant accuracy to minimize its eccentricity.
In light of this, there has been proposed and already put to practical use a system which moves optics with an object lens bodily in the radial direction, instead of the object lens only. In a record/playback head driven under such an alternative control, the object lens needs only to move along the optical axis of the optics, or in the focal direction, effectively eliminating displacement of the optical axis.
However, in order to attain an accurate tracking control by the radial movement of the whole optics on the head, not only a carriage supporting the head has to be assembled with accuracy, but also a mechanism for driving the carriage in the radial direction has to be highly accurate. Typical of such drive mechanisms heretofore known is one which employs tracks or rails extending in the radial direction of a disc along which a record/playback head moves. A carriage is movably supported by the tracks through linear bearings. Again, the linear bearings have to be machined with an accuracy of the order of submicrons in order to achieve accurate tracking control. Additionally, such a drive mechanism is not fully acceptable due to the problems concerned with the assembly of the linear bearings on the tracks, scratches or wear tending to occur on their interengaged surfaces, etc.